1. Field of the Invention
The present invention relates to a flow control valve which is provided in a cooling system for cooling an engine by circulating cooling water through the engine and which is used for controlling a flow quantity of the cooling water.
2. Description of Related Art
Cooling systems of a water cooling type conventionally used in engines have generally been arranged to control cooling water at a constant temperature of about 80° C. by means of a thermostat without reference to an operating state of the target engine. However, changing a cooling degree of an engine according to an operating state (a loaded condition, a rotational speed, etc.) of the engine was found to be effective in reducing friction of the engine, improving fuel efficiency, enhancing knocking performance, and preventing the overheating of the cooling water. Accordingly, there have been proposed several types of cooling systems using cooling water each arranged to control a cooling degree of an engine according to an operating state of the engine.
Such cooling systems of engines are disclosed in Japanese patent unexamined publications Nos. 09(1997)-195768 and 2000-18039. The cooling system disclosed in the JP unexamined publication No. 09(1997)-195768 is provided with a flow control valve including a first valve body and a first valve seat for controlling a flow quantity of the cooling water which flows out of an engine and returns to a water pump by way of a radiator (hereinafter referred to as a “radiator flow quantity”), a second valve body and a second valve seat for controlling a flow quantity of the cooling water which flows out of the engine and bypass the radiator to directly return to the water pump (hereinafter referred to as a “bypass flow quantity”), and an electromagnetic actuator which drives the first and second valve bodies integrally as a valve unit. The above electromagnetic actuator is constructed of an electromagnetic coil which attracts a shaft made of a magnetic material when electric current is applied to the coil, thereby displacing the shaft downward against the force of a spring. Upon stop of the application of electric current to the coil, on the other hand, the shaft is displaced upward by the force of the spring. In association with the shaft displacement, the first and second valve bodies are driven together as a valve unit.
Similar to the above cooling system disclosed in JP unexamined publication No. 9(1997)-195768, the cooling system disclosed in JP unexamined publication No. 2000-18039 is provided with a radiator circuit for permitting cooling water which flows out of an engine to circulate through a radiator and a bypass circuit for permitting the cooling water which flows out of the engine to bypass the radiator to flow back to the engine. In a portion at which the bypass circuit and the radiator circuit meet, there is disposed a rotary flow control valve for controlling a flow quantity (the radiator flow quantity) of the cooling water flowing in the radiator circuit and a flow quantity (the bypass flow quantity) of the cooling water flowing in the bypass circuit. This flow control valve includes a rotary valve having a cup shape rotatably provided in a housing. This flow control valve is constructed to measure the radiator flow quantity and the bypass flow quantity at an outer periphery of the rotary valve and cause the cooling water flowing in the radiator circuit and the bypass circuit to flow together to return to the engine through a pump.
And now, in the above flow control valve disclosed in JP unexamined publication No. 9(1997)-195768, at the time of driving the valve by operation of the electromagnetic actuator, this actuator is required to produce a driving torque enough to overcome the force of the spring, the force of pressure of the cooling water, and the force caused by collision of the cooling water with each valve. The first valve body is acted upon by the pressure of fluid at an inlet port of the flow control valve (namely, a radiator flow inlet pressure), while the second valve body is acted upon by the pressure of fluid at another inlet port of the flow control valve (namely, a bypass flow inlet pressure). Thus, a difference between those two pressures acts on a valve unit. If the pressure difference is large, the thrust corresponding to the difference is applied to the valve and therefore the electromagnetic actuator is requested to produce a large driving torque. In general, the diameter of a passage for the bypass flow (hereinafter referred to as a “bypass passage”) is smaller than that of a passage for the radiator flow (hereinafter referred to as a “radiator passage”). When the bypass flow quantity becomes larger than the radiator flow quantity, the pressure in the bypass passage becomes a negative pressure, resulting in a large influence on a pressure characteristic. Accordingly, bypass flow inlet pressure is largely reduced depending on a bypass flow quantity characteristic, thereby increasing the pressure difference mentioned above. As a result, the electromagnetic actuator is required to produce a large driving torque to open the flow control valve against the thrust resulting from the pressure difference. This leads to a need to upsize the actuator, which may cause problems of a deterioration in mountability of the flow control valve with respect to the engine and an increase in manufacturing cost of the flow control valve.
In the flow control valve disclosed in JP unexamined publication No. 2000-18039, on the other hand, there is a need to measure the radiator flow quantity and the bypass flow quantity at the outer periphery of the rotary valve. Furthermore, many cooling systems currently used adopt “an internal bypass type” which is provided with a bypass circuit in the inside of an engine block to flow cooling water through the bypass circuit. Accordingly, the flow control valve disclosed in JP unexamined publication 2000-18039 could not directly be used in the internal bypass type of cooling system. To adopt the flow control valve, there is a need to change the shape of the engine or to additionally provide a bypass pipe to the outside of the engine block. Consequently, the cost of manufacturing the cooling system would be increased extremely.